This invention relates to a method of, and apparatus for, controlling the temperature of pressurized air supplied from a source thereof, for use in a system or a device which makes use of it.
The invention has been devised in the context of the supply of bleed air from at least one gas turbine engine of an aircraft. It is known to use bleed air from a gas turbine engine (i.e. air taken from the flow of air through the engine after a compressor stage of the engine and thus pressurized) in ancillary systems of the aircraft such as an air conditioning system for example. A further example where bleed air might be used in an aircraft is a system for oxygen separation using a pressure swing adsorption process, to provide an oxygen or oxygen-enriched supply for breathing by crew or other persons on board the aircraft.
Some systems such as pressure swing adsorption systems for oxygen separation are particularly sensitive to variations in the temperature of pressurized air with which they are supplied, and the temperature must be controlled within relatively narrow limits if the system is to perform satisfactorily. Bleed air as supplied from an aircraft engine(s) is usually at a higher temperature than that required, and consequently needs to be cooled which is usually accomplished by passing the bleed air through a heat exchanger which puts it in heat exchange relationship with a coolant fluid. The cooling effect of a heat exchanger depends on the difference in temperature between the fluid which is being cooled and the coolant fluid, and on the flow rates of the two fluids. Often the coolant is ram air, i.e. ambient air which is caused to pass through the heat exchanger as a result of the aircraft's forward movement and it will be appreciated that in this case the temperature and flow rate of the coolant air can vary substantially: such variations result in variations in the temperature of the bleed air after it has passed through the heat exchanger. If the coolant is ram air, none is provided if the aircraft is not moving.